1. Field of the Invention
The present invention relates to an optical block which is suited for use in optical systems of liquid crystal projectors and so on.
2. Description of the Related Art
Displays such as projectors, television receivers and displays for computers which use light modulation means, for example, liquid crystal panels prevail nowadays in a variety of fields.
A three-panel type liquid crystal projector, for example, polarizes RGB light emitted from a light source such as a metal halide lamp or a halogen lamp into a predetermined polarized wavefront with a polarizing splitter block and then decomposes the RGB light into, R, G and with the appropriate B colors with optical elements such as dichroic mirrors. After incidence on liquid crystal panels corresponding to the colors and light modulation, the colors are composed with a dichroic block, whereby RGB light which is a colored image can be obtained as output light. The composed colored image is projected through a projector lens to a screen.
By the way, light which is emitted from an ordinary light source has two kinds of wavefronts, which can generally be classified into a P polarized component (hereinafter referred to as P wave) and an S polarized component (hereinafter referred to an S wave). Disposed in a liquid crystal projector such as that described above is a polarizing splitter block at a location before the light emitted from the light source is allowed to be incident onto the liquid crystal panels for irradiation with rays which have either of the polarized wavefront of the P wave or the S wave corresponding to polarizing plates disposed on front surfaces of the liquid crystal panels.
It is regarded that the polarizing splitter block makes it possible to effectively utilize light emitted from light sources and is effective for composing liquid crystal projectors which are to provide images having high luminance in particular.
FIG. 1 is a perspective view exemplifying a polarizing splitter block.
As shown in this drawing, a polarizing splitter block 52 is composed by cementing a plurality of prisms 52a through 52f which are made of glass: the prisms 52b and 52e constitutes composing sections of incidence, and prisms 52a, 52c, 52d and 52f constitutes composing sections of emergence. Disposed between the prisms 52b and 52c and between the prisms 52d and 52e are polarizing beam splitters (hereinafter referred to simply as PBSs) 53a and 53b which transmit the P wave and reflect the S wave, for example, whereby the P wave and the S wave are separated once by these polarizing beam splitters.
Since the prisms 52b and 52e or the prisms 52c and 52d are made of glass and have excellent heat resistance, these PBSs 53a and 53b are formed as multi-layer thin films by a method, for example, of vapor deposition or sputtering.
The prisms 52a and 52f are composed as reflecting prisms and have multi-layer films of a dielectric material formed on outside surfaces thereof which have an angle of 45-degree relative to an optical axis of a light source which is not shown in this drawing. Furthermore, wavelength plates 54a and 54b are disposed on the emergence sides of the polarizing splitter block 52 at locations before the multi-layer films of the dielectric material for polarizing incident S wave into the P wave.
Light that is polarized, for example, into the P wave by the polarizing splitter block 52 described above is made monochromatic into the three primary colors RGB as described above, subjected to light modulation and the composed, for example, by a dichroic block 60 such as that shown in FIG. 2.
The dichroic block 60 shown in FIG. 2 has an outside form composed by cementing end surfaces of four prisms 61a, 61b, 61c and 61d which are made of glass. On predetermined end surfaces of these prisms 61a to 61b, dichroic filters 62a and 62b are formed as multi-layer films, for example, by vapor deposition.
A red ray R which is incident from the prism 61a, for example, is reflected by the dichroic filter 62a, whereas a green ray G which is incident from the prism 61c is reflected by the dichroic filter 62b. A blue ray B which is incident from the prism 61b transmits through the dichroic filters 62a and 62b, whereby the rays of the three colors are composed and emerge as a colored image from the prism 61d.
The prisms 52a to 52f and the prisms 61a to 61d which compose the polarizing splitter block 52, the dichroic block 60, etc. are generally made of glass. When the prisms and the polyhedrons are made of glass, however, they require considerable working time and cost since it is necessary to polish end surfaces thereof after they are formed. Furthermore, weights of the polarizing splitter block 52 and the dichroic block 60 are increased by using glass as a material of the prisms, thereby making the projector itself heavier.
Furthermore, the PBSs 53a and 53b the dichroic filters 62a and 62b are formed on the prisms by vapor deposition, sputtering or a similar method and it is necessary to prevent a material of the films from turning to surfaces other than target surfaces when vapor deposition or sputtering is carried out on the polyhedrons.
For reducing the weights, it is considered to make the prisms 52a to 52f and the prisms 61a to 61d of a plastic material. In view of a problem of heat resistance, however, it is difficult to form the PBSs 53a and 53b and the dichroic filters 62a and 62b by vapor deposition or sputtering though the plastic material can be molded with metal dies at a low cost.
Moreover, it is possible to reduce the turning of the material of films and further reduce the weights by forming the PBSs 53a and 53b and the dichroic filters 62a and 62b on plate-like glass members without using the prisms 52a to 52f and the prisms 61a to 61d, but optical path distances are made longer than those obtained by disposing the prisms when only the plate-like glass members are used. Accordingly, there is posed a problem that an optical system is enlarged, thereby enlarging the projector itself.